Continuously variable valve timing system and method for controlling the same

ABSTRACT

A continuously variable valve timing (CVVT) system which may be operated in cooperation with a continuously variable valve lift (CVVL) engine may be included. A reference position of a suction CVVT may be set by a spring. In addition, a method for controlling a continuously variable valve timing (CVVT) system may be included. The method includes setting a reference position of a suction CVVT to a most advanced angle position, and controlling a delayed angle amount at the reference position of the most advanced angle.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority to Korean Patent Application No.10-2011-0063011, filed on Jun. 28, 2011 in the Korean IntellectualProperty Office, the entire contents of which is incorporated herein forall purposes by this reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a continuously variable valve timingsystem and a method for controlling the same, and more particularly, toa continuously variable valve timing system and a method for controllingthe same which can improve a fuel efficiency by controlling a referenceposition when a general hydraulic CVVT (continuously variable valvetiming) system is employed in a continuously variable valve lift (CVVL)engine having no self-advanced/delayed angle function.

2. Description of Related Art

In general, it is known in CVVL engines that a valve lift and valveopening duration is directly controlled by an electronic control unit(ECU) to improve fuel efficiency, performance and responsibility, andreduce emissions.

A valve lift changing property of the CVVL system is varied dependingupon the structure of the variable lift mechanism of the CVVL engine. Inview of the engine control, the valve lift changing property can bedivided into two types described below, depending upon whether themaximum opening point (MOP) is varied or not when the valve lift ischanged.

FIG. 1 is a view illustrating a lift changing property of a CVVL systemhaving no self-advanced/delayed angle function. FIG. 2 is a viewillustrating a lift changing property of a CVVL system having aself-advanced/delayed angle function. FIGS. 1 and 2 show the change insuction valve lift profile at a valve lift variation according to thepresence or absence of the self-advanced/delayed angle function.

The CVVL system having no self-advanced/delayed angle function is asystem in which the MOP is not changed by the operation of the CVVLmechanism when the valve lift is changed. The structure thereof isrelatively simple, which is preferable to CVVL embodiment, but the camtiming is controlled by a separate CVVT system. The CVVL system havingthe self-advanced/delayed angle function is a system in which the MOP ischanged by the operation of the CVVL mechanism when the valve lift ischanged. If the advanced/delayed angle property of the CVVL mechanism isutilized, it can share a portion of separate CVVT operating necessity.

In addition, it is necessary to perform a cam timing control bymonitoring a separate CVVT system to the CVVL engine having noself-advanced/delayed angle function. A conventional hydraulic CVVTsystem which is generally employed in a common Non-CVVL engine is used.

In the hydraulic CVVT system for the Non-CVVL engine, since a fixedsuction value lift is used in the Non-CVVL engine, a suction/exhaust camtiming is controlled to minimize a fuel consuming amount under drivingconditions. In the case of the general Non-CVVL engine, a referenceposition is selected on the basis of the optimum cam timing in alow-speed and low-load region including start and idle, and generallycorresponds to a suction/exhaust timing at which the value overlap isminimized.

That is, the reference position of the suction cam timing is the mostdelayed angle position, while the reference position of the exhaust camtiming is the most advanced angle position. The reference positionshould be maintained under a condition in which the oil pressuresufficient for driving the CVVT is not generated at start and low-speeddriving.

In the CVVT, as shown in FIG. 3, if an outer sprocket rotates, an innerrotor rotates slower than the outer sprocket due to cam frictionaltorque, and thus the timing is always shifted to the most delayed angleposition. In the case of the suction CVVT, the shifted position becomesthe reference position, but in the case of the exhaust CVVT, a biasspring is installed between the sprocket and the inner rotor toforcefully shift the timing to the most advanced angle position, so thatthe most advanced angle position is maintained through the resilientforce. If RMP is increased or the idle is increased as compared with theidle driving condition, the optimum cam timing should be shifted in sucha manner that the suction is shifted to the advanced angle direction andthe exhaust timing is shifted to the delayed angle direction, inrelation to the idle cam timing. In this instance, the oil pressurecreated in a head oil gallery is respectively applied to the advancedangle chamber and the delayed angle chamber in the CVVT apparatus by anoil control valve (OCV) through two oil circuits (advanced angle oilpassage and delayed angle oil passage). In the case of the camfrictional torque and the exhaust CVVT, the CVVT is operated against theresilient force of the bias spring by the pressure difference betweenthe advanced angle chamber and the delayed angle chamber.

In the case of the hydraulic CVVT employed in general Non-CVVL engines,an oil pressure of a predetermined level or more should be obtained tooperate the CVVT. Accordingly, since the stability and responsibility inthe CVVT control is significantly deteriorated at the low speed (usuallyidle RPM) which cannot create the sufficient oil pressure in theNon-CVVL engine, the cam timing of the reference position (suction themost delayed angle position and the exhaust most advanced angleposition) should be used as it is. Therefore, the cam timing is selectedas the reference position, and the cam timing is controlled in such amanner that the suction is shifted in the advanced angle direction andthe exhaust is shifted in the delayed angle direction, rather than thereference position, in the engine driving region except for thelow-speed and low-load duration.

However, in the case where the CVVT system for the Non-CVVL engine isemployed in the CVVL engine having no self-advanced/delayed anglefunction, it is not possible to control the optimum cam liming, and thusthere are some problems of reducing the fuel efficiency, generatingknocking, decreasing vehicle responsibility, and vibrating the vehicle.

That is, only when the RPM is increased to create the oil pressuresufficient to operate the CVVT, the suction/exhaust cam timing can bevaried. However, in the low-speed duration the CVVT is not operated dueto the lack of the oil pressure, and the suction/exhaust CVVT should beoperated in the reference position (suction most delayed angle andexhaust most advanced angle). In the case of using the lowest delayedlift, there is another problem in that the fuel efficiency isdeteriorated due to the increased pumping loss and the increasedeffective compression ratio.

The information disclosed in this Background of the Invention section isonly for enhancement of understanding of the general background of theinvention and should not be taken as an acknowledgement or any form ofsuggestion that this information forms the prior art already known to aperson skilled in the art.

BRIEF SUMMARY

Various aspects of the present invention are directed to providing acontinuously variable valve timing system and a method for controllingthe same, which improves a method for controlling selection of areference position of a CVVT (continuously variable valve timing) in ageneral Non-CVVL engine and a CVVL (continuously variable valve lift)engine when a general hydraulic CVVL system is employed in the CVVLengine having no self-advanced/delayed angle function, thereby reducinga pumping loss and thus improving a fuel efficiency and an effectivecompression ratio to improve a knock property and reduce variations ofRPM in a cycle.

In one aspect of the present invention, there is provided a method forcontrolling a continuously variable valve timing (CVVT) system which isoperated in cooperation with a continuously variable valve lift (CVVL)engine, including the steps of, setting a reference position of asuction CVVT to a most advanced angle position. controlling a delayedangle amount at the reference position of the most advanced angle.

With the above configuration, the present invention can set a low valvelift and optimum cam timing, which is necessary for the same in a lowRPM region including an idle condition, as a reference position.Therefore, the optimum valve lift can be used in the region withoutemploying the CVVT control, thereby improving the fuel efficiency in theidle and low-speed region. In addition, a proper effective compressionratio is maintained to suppress generation of knock and decreasevariations of RPM. When the valve lift is changed, it is possible toimprove the fuel efficiency at the low speed, without complicating themechanism required to employ the CVVL mechanism having theself-advanced/delayed angle function and increasing rotational inertialand friction, as well as a cost. Furthermore, as compared with the CVVLmechanism having the self-advanced/delayed angle function, when atemperature of cooling water is low or atmosphere is low, thecombination of the valve lift and the cam timing optimized for variousdriving conditions, such as full-load driving and costing driving, canbe achieved by using the general hydraulic CVVT module to improve amerchantable quality.

The methods and apparatuses of the present invention have other featuresand advantages which will be apparent from or are set forth in moredetail in the accompanying drawings, which are incorporated herein, andthe following Detailed Description, which together serve to explaincertain principles of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view illustrating a lift changing property of a CVVL systemhaving no self-advanced/delayed angle function.

FIG. 2 is a view illustrating a lift changing property of a CVVL systemhaving a self-advanced/delayed angle function.

FIG. 3 is a view illustrating the structure of a hydraulic CVVT systemin the related art.

FIG. 4 is a view illustrating the structure of a hydraulic CVVT systemaccording to an exemplary embodiment of the present invention.

FIG. 5 is a graph illustrating the efficiency based on a valve lift anda suction cam timing.

FIG. 6 is a view illustrating an optimum valve lift and a suction timingin a continuously variable valve lift engine.

FIG. 7 is a view illustrating the optimum valve lift and the suctiontiming for each driving condition.

FIG. 8 is a view illustrating properties of the optimum valve lift andthe suction timing.

FIG. 9 is a view illustrating an effective compression ratio and a RAMvariation according to a lift/suction cam.

It should be understood that the appended drawings are not necessarilyto scale, presenting a somewhat simplified representation of variousfeatures illustrative of the basic principles of the invention. Thespecific design features of the present invention as disclosed herein,including, for example, specific dimensions, orientations, locations,and shapes will be determined in part by the particular intendedapplication and use environment.

In the figures, reference numbers refer to the same or equivalent partsof the present invention throughout the several figures of the drawing.

DETAILED DESCRIPTION

Reference will now be made in detail to various embodiments of thepresent invention(s), examples of which are illustrated in theaccompanying drawings and described below. While the invention(s) willbe described in conjunction with exemplary embodiments, it will beunderstood that the present description is not intended to limit theinvention(s) to those exemplary embodiments. On the contrary, theinvention(s) is/are intended to cover not only the exemplaryembodiments, but also various alternatives, modifications, equivalentsand other embodiments, which may be included within the spirit and scopeof the invention as defined by the appended claims.

Hereinafter, a preferred embodiment of the present invention will bedescribed with reference to the accompanying drawings. In the entiredescription of the present invention, the same drawing referencenumerals are used for the same elements across various figures.

FIGS. 4 to 9 are views illustrating a continuously variable valve timingsystem and a method of controlling the same according to an exemplaryembodiment of the present invention. FIG. 4 is a view illustrating thestructure of a hydraulic CVVT system according to an exemplaryembodiment of the present invention. FIG. 5 is a graph illustrating theefficiency based on a valve lift and a suction cam timing. FIG. 6 is aview illustrating an optimum valve lift and a suction timing in acontinuously variable valve lift engine.

In addition, FIG. 7 is a view illustrating the optimum valve lift andthe suction timing for each driving condition. FIG. 8 is a viewillustrating properties of the optimum valve lift and the suctiontiming. FIG. 9 is a view illustrating an effective compression ratio anda RAM variation according to a lift/suction cam.

In a CVVL engine, controlling a dimension of a valve lift continuouslyand variably is to minimize a pumping loss and thus improve a fuelefficiency. Since the valve lift and a valve opening duration can bevariably controlled in the CVVL engine, the engine can be operated byselecting a suction valve opening position and a suction valve closingposition to have an optimum value in cooperation with a CVVT mechanism.

As shown in FIGS. 5 and 6, if the suction valve closing timing issignificantly shifted to an advanced angle, the air confined in acombustion chamber is adiabatically expanded to a bottom dead center(BDC), and then is subjected to adiabatic compression as a piston israised.

In this instance, since the pumping loss is theoretically zero to apiston position during a suction valve closing timing, the pumping lossis minimized by suctioning an air volume corresponding to a volume of acombustion chamber for the suction valve closing duration. Meanwhile, inthe case where the suction valve closing position is significantlyshifted to an advanced angle in the general Non-CVVL engine having aconstant valve lift and valve closing direction, the suction valveopening timing is significantly shifted to the advanced angle, and thusthe valve overlap becomes excessively large, so that misfire occurs dueto unstable combustion, which is a condition which cannot drive avehicle.

Meanwhile, in FIG. 5, a first diagram indicates a combustion pressureand a volume at a maximum lift (valve lift which is equal to MP1), and asecond diagram indicates a combustion pressure and a volume at a minimumlift (throttle minimizing lift by a throttle body).

In comparison to the pumping loss (area of a border portion of the firstdiagram) in the case of using the maximum lift, the pumping loss (areaof red border portion) in the case of using the minimum lift is varieddepending upon the minimum lift, but when the minimum lift is 1 mm, thepumping loss can be reduced by ⅓ of the Non-CVVL (or the maximum lift).It can reduce an indicated mean effective pressure (IMEP) and fuelconsumption, thereby improving the fuel efficiency.

FIGS. 7 and 8 show the optimum valve lift and a suction timing property,in which the below engine driving property is obtained according to thelift and the suction cam.

First, if the lift is minimum and the suction cam is the most advancedangle, the pumping loss is minimized and the fuel efficiency isimproved. If the lift is minimum and the suction cam is at the mostdelayed angle, the effective compression ratio is excessive, knockingoccurs, and an RPM variation in the cycle is excessive. If the lift ismaximum and the suction cam is the most advanced angle, the inner EGR isexcessive, and the combustion stability is deteriorated. If the lift ismaximum and the suction cam is at the most delayed angle, the high-speedoutput is increased.

The properties of the continuously variable valve timing for each loadregion will now be described.

Since the fuel efficiency is important in the idle region including alow speed and a low load, the minimum lift and the most advanced anglesuction cam timing should be used. In the engine equipped with the oilpump and the CVVT system for a general Non-CVVL, since the oil pressuresufficient to operate the CVVT is not created due to the low RPM, thesuction cam timing is fixed to the most delayed angle condition. In thisinstance, the pumping loss can be reduced by using the maximum liftinstead of the minimum lift, but the fuel efficiency is deteriorated incomparison with the minimum lift and the most advanced angle suctioncam.

With reference to the idle silence, a torque difference required for thecompression process according to the effective compression ratio occurs,and thus there is a difference in the RPM variation properties within acycle. In general, if the effective compression ratio is lowered at theidle driving, the RPM variation is decreased to improve the idlesilence. Accordingly, as shown in FIG. 9, it can be known that theminimum lift and the most advanced angle condition which lowers theeffective compression ratio are advantageous in order to reduce the idleRPM variation.

The minimum lift and the most advanced angle suction cam timing arerequired in a partial-load region which is an important region, as wellas the idle region. In this instance, as the load is increased, the liftis increased and the suction cam timing is slightly delayed.

In the full-load region, it is important to improve a torque by suctionthe maximum air volume at the full load, and thus a lift capable ofobtaining the maximum air volume, and a cam timing at that time areselected. In general, 60% to 80T of the maximum lift at low and middlespeed is used, and the maximum lift is used at the high speed.

Since there are the suction valve lift and the suction valve timingwhich can maximize the fuel efficiency and the combustion stabilitydepending upon the driving condition, the CVVL engine having aself-advanced angle function has an effect of obtaining the advancedangle of the opening/closing timing and a valve profile throughreduction of the lift, but the CVVL engine having no self-advanced anglefunction should have the optimum cam timing by the control and operationof the CVVT.

The CVVT system capable of obtaining the optimum cam timing in the CVVLengine according to an exemplary embodiment of the present invention,and the method for controlling the same will be described.

As described above, the optimum fuel efficiency, drivability andperformance can be satisfied by simultaneously obtaining the optimumlift and cam timing for every engine driving region. It can be seen thatCVVT requirements at the low speed are different from those of thegeneral Non-CVVL engine.

Therefore, the present invention relates to the hydraulic control CVVTfor driving the optimum cam timing for every valve lift in the CVVLengine having no self-advanced/delayed angle function or remarkablyinsufficient advanced/delayed angle amount, as compared with thenecessary advanced/delayed angle amount, when the valve lift is varied.A reference position of the suction CVVT is set to the most advancedangle position, and a CVVT assembly with a bias spring to control thedelayed angle amount at the reference position of the most advancedangle.

According to an exemplary embodiment of the present invention, thereference position of the suction CVVT in the CVVT system which operatesin cooperation with the CVVL engine is set by the spring.

In order to set the reference position of the suction CVVT to the mostadvanced angle position and control the delayed angle of the CVVT, theCVVT system is provided with the bias spring so that the suction camtiming is shifted in an advanced angle direction when an oil controlvalve installed in the CVVT system does not operate (when PWM is zero).In the case where the oil control valve installed in the CVVT systemoperates to the max (when PWM is applied), it is preferable that the oilcontrol valve changes an oil pressure passage connected to the CVVTsystem to change an oil circuit in a head or an oil passage in a camshaft.

As described above, in the state in which the oil control valve does notoperate or the head oil pressure is not sufficient at the low speed,since the oil pressure is not applied to the advanced angle chamber andthe delayed angle chamber in the CVVT module, the cam timing is shiftedto the advanced angle position by the resilience force of the biasspring.

The position of the CVVT can be fixed by using a lock pin, like ageneral CVVT module. If the oil control valve is operated after the oilpressure is created, the oil pressure is applied to the delayed anglechamber of the CVVT module through the oil control valve and the oilcircuit. The delayed angle chamber is connected to a drain passagethrough the oil control valve, and thus the cam timing is shifted in theadvanced angle direction by the oil pressure difference in the advancedangle chamber and the delayed angle chamber.

With the continuously variable valve timing system and the controlmethod according to an exemplary embodiment of the present invention,the reference position of the suction CVVT is set to the most advancedangle position in the CVVT system which is operated in cooperation withthe CVVL engine. After that, the delayed angle amount is controlled atthe reference position of the advanced angle to improve the fuelefficiency at the low speed and increase the merchantable quality.

The foregoing descriptions of specific exemplary embodiments of thepresent invention have been presented for purposes of illustration anddescription. They are not intended to be exhaustive or to limit theinvention to the precise forms disclosed, and obviously manymodifications and variations are possible in light of the aboveteachings. The exemplary embodiments were chosen and described in orderto explain certain principles of the invention and their practicalapplication, to thereby enable others skilled in the art to make andutilize various exemplary embodiments of the present invention, as wellas various alternatives and modifications thereof. It is intended thatthe scope of the invention be defined by the Claims appended hereto andtheir equivalents.

1. A continuously variable valve timing (CVVT) system which is operatedin cooperation with a continuously variable valve lift (CVVL) engine,wherein a reference position of a suction CVVT is set by a spring. 2.The continuously variable valve timing (CVVT) system of claim 1, whereinthe spring is adapted such that a suction cam timing is shifted in anadvanced angle direction when an oil control valve installed in the CVVTsystem does not operate.
 3. A method for controlling a continuouslyvariable valve timing (CVVT) system which is operated in cooperationwith a continuously variable valve lift (CVVL) engine, comprising thesteps of: setting a reference position of a suction CVVT to a mostadvanced angle position; and controlling a delayed angle amount at thereference position of the most advanced angle.
 4. The method accordingto claim 3, wherein when an oil control valve installed in the CVVTsystem does not operate, a suction cam timing is shift in an advancedangle direction by a bias spring which is installed in the CVVT system.5. The method according to claim 3, wherein when the oil control valveinstalled in the CVVT system operates to the max, the oil control valvechanges an oil pressure passage connected to the CVVT system.